As a blue-violet semiconductor laser has been put into practical use, a Blu-ray disc (hereafter BD)—an optical information recording medium (hereafter optical disk)—having high density and large capacity as well as the same size as a CD (Compact Disc) and the DVD (Digital Versatile Disc), has been commercialized. BD is an optical disk for recording or reproducing information on or from an information recording surface of which thickness of the light transmission layer is approximately 0.1 mm, using a blue-violet laser source which emits a laser beam having approximately a 400 nm wavelength, and an objective lens of which numerical aperture (NA) is approximately 0.85.
An objective lens made from synthetic resin is normally used for an optical head for recording or reproducing information on or from such an optical disk as a CD or a DVD. Compared with a glass objective lens, the specific gravity of a synthetic resin objective lens is smaller. Therefore if a synthetic resin objective lens is used, burden on an objective lens actuator, which drives an objective lens with respect to axial run-out and decentering of an optical disk, can be decreased, and the objective lens can follow up the axial run-out and decentering of the optical disk at high-speed. Furthermore, a synthetic resin objective lens can be mass produced at high precision by injection molding, therefore cost of the objective lens can be decreased.
Hence a synthetic resin objective lens is often used even for a high NA objective lens that is used for a BD optical head.
Many types of so-called compatible objective lenses are also known, where a diffraction structure is formed on the synthetic resin objective lens whereby spherical aberration, generated by the difference of thickness of the light transmission layer among a plurality of types of optical disks, is corrected using the difference of the light source wavelength.
For example, Patent Literature 1 discloses an objective lens, which generates a function of a convex lens by diffracting the blue-violet laser beam having the wavelength λ1, so as to implement convergence on the information recording surface of the BD of which thickness of the light transmission layer is approximately 0.1 mm, and which generates a function of a concave lens by diffracting the red laser beam having the wavelength λ2, so as to implement convergence on the information recording surface of the DVD of which thickness of the light transmission layer is approximately 0.6 mm.
FIG. 19 is a diagram depicting a configuration of a conventional objective lens. The left drawing in FIG. 19 is a schematic plan view depicting a configuration of a conventional objective lens 90, and the right drawing in FIG. 19 is a schematic cross-sectional view depicting the configuration of the conventional objective lens 90. A zonal diffraction structure (hologram) is formed on the entrance surface 91 on the light source side (side where laser beam enters) of the objective lens 90, centering around the optical axis OA of the objective lens 90. The diffraction structure is different between the inner circumference area 911 including the optical axis OA and the outer circumference area 912 which is a peripheral area of the inner circumference area 911.
The inner circumference area 911 is a compatible area which is used for recording or reproducing the DVD using a red laser beam, and recording or reproducing the BD using a blue-violet laser beam, and is designed such that plus first-order diffracted light of the blue-violet laser beam is converged on the information recording surface of the BD and minus first-order diffracted light of the red laser beam is converged on the information recording surface of the DVD.
On the other hand, the NA upon recording or reproducing information on or from the BD using the blue-violet laser beam (approximately 0.85) is greater than the NA upon recording or reproducing information on or from the DVD using a red laser beam (approximately 0.60). Therefore it is designed such that the outer circumference area 912 is an area dedicated to the BD, and only the blue-violet laser beam is converged on the information recording surface of the BD, and the red laser beam generates an aberration on the information recording surface of the DVD.
One unit of the step difference of the diffraction structure of the inner circumference area 911 is an amount to generate approximately a 1.25×λ1 [nm] of optical path difference for the blue-violet laser beam having the wavelength λ1 (λ1=405 nm), and the phase modulation amount is π/2 per step. In this case, the diffraction efficiency of the plus first-order diffracted light is approximately 80% based on the scalar calculation, which is highest among the orders of diffraction.
One unit of the step difference of the diffraction structure in the inner circumference area 911 is an amount to generate approximately a 0.75×λ2 [nm] of optical path difference for the red laser beam having the wavelength λ2 (λ2=660 nm), and the phase modulation quantity −π/2 per step. In this case, the diffraction efficiency of the minus first-order diffracted light is approximately 80% based on the scalar calculation, which is highest among the orders of diffraction.
If the inner circumference area 911 has this diffraction structure, a compatible recording or compatible reproduction of information can be implemented at high light utilization efficiency for the DVD having a 0.6 mm thick light transmission layer, and for the BD having approximately a 0.1 mm thick light transmission layer.
If a lens having a diffraction structure, which is not limited to a compatible objective lens, is used, the diffraction efficiency may change depending on the radius position of the lens. This is because the pitch of the diffraction structure in the effective diameter of the lens is different depending on the radius position. Generally if the lens power is generated by the diffraction structure, the pitch of the diffraction structure decreases and diffraction efficiency decreases as the radius position approaches from the inner circumference near the optical axis to the outer circumference.
The intensity distribution of the semiconductor laser used for the optical head for the optical disk, on the other hand, decreases Gaussian-functionally as the distance from the optical axis increases, hence the intensity of the laser beam is lower in the outer circumference than in the inner circumference. If the intensity of the laser beam, which enters the objective lens, decreases dramatically in the outer circumference, the effective NA of the objective lens decreases. As a result, the focal spot on the information recording surface of the optical disk cannot be sufficiently focused.
With the foregoing in view, Patent Literature 2, for example, discloses a configuration of an optical head that uses an objective lens having a diffraction structure, wherein a light distribution correction element for decreasing a predetermined amount of transmittance in an area near the optical axis is disposed in order to correct a drop in the intensity of a laser beam that enters the objective lens, generated along with the increase in the distance from the optical axis, and to suppress the deterioration of focal spots on an information recording surface of the optical disk.
Furthermore according to the conventional objective lens disclosed in Patent Literature 1, for example, the inner circumference area is used for recording or reproduction for both the DVD and the BD. Therefore the pitch of the diffraction structure becomes small in the area near the outermost circumference of the inner circumference area, and there may be a case where diffraction efficiency drops.
FIG. 20 is a graph depicting a diffraction efficiency of the conventional objective lens. In FIG. 20, in the conventional objective lens 90 disclosed in Patent Literature 1, the diffraction efficiency of the blue-violet laser beam having the wavelength λ1 and that of the red laser beam having the wavelength λ2 are calculated based on the wave calculation (vector calculation). In FIG. 20, the abscissa is the entrance position of the laser beam, that is the distance from the optical axis OA (radius of objective lens), and the ordinate is the diffraction efficiency corresponding to the entrance position.
As FIG. 20 shows, in the case of the wavelength λ2, the diffraction efficiency is 70% or more in a position near the optical axis in the inner circumference area (point α), whereas the diffraction efficiency drops to 50% or less in a position near the outermost circumference of the inner circumference area (point β). This is because the pitch of the diffraction structure is smaller in the position of the point β than the position of the point α, and in addition, the inclination angle of the entrance surface 91 increases and the incidence angle of the laser beam which enters the objective lens 90 in parallel increases as the distance from the optical axis OA increases. The diffraction efficiency in the position of the point β can further drop by the dispersion in molding, for example.
If the intensity of the laser beam which enters the objective lens drops dramatically in the outer circumference, as mentioned above, the focal spot on the information recording surface of the optical disk cannot be sufficiently focused, but this problem of the conventional compatible objective lens is not mentioned in Patent Literature 1.
Further, Patent Literature 2 discloses a configuration to dispose the light distribution correction element for decreasing a predetermined amount of transmittance in an area near the optical axis, in order to suppress the deterioration of focal spots on the information recording surface of an optical disk, but here an optimum configuration of the above mentioned compatible objective lens is neither disclosed nor suggested.